At present, a honeycomb catalyst where a catalyst is carried on a honeycomb structure is used in purifying exhaust gas discharged from various kinds of engines. As shown in FIG. 7, this honeycomb catalyst has a structure in which a catalyst layer 15 is carried on the surface of partition walls 4 forming cells 3. Moreover, as shown in FIGS. 8, 9, to purify the exhaust gas by the use of a honeycomb catalyst 60 (a honeycomb structure 11), the exhaust gas is allowed to flow into the cells 3 of the honeycomb catalyst 60 from one end face 2a side, brought into contact with catalyst layers (not shown) on the surfaces of partition walls 4, and discharged from the other end face 2b side (e.g., see Patent Document 1).
In the case of purifying the exhaust gas by the use of such a honeycomb catalyst, conveyance of components to be purified contained in the exhaust gas from the exhaust gas to the catalyst layer on the surfaces of the partition walls needs to be promoted as much as possible to improve a purification efficiency. To improve the purification efficiency of the exhaust gas, reduction of a hydraulic diameter of the cells, enlargement of a surface area of the partition walls and the like are necessary. Specifically, a method for increasing the number of cells per unit area (a cell density) and the like are employed.
Here, it is known that a conveyance ratio of the components to be purified from the exhaust gas to the catalyst layer on the surfaces of the partition walls increases in inverse proportion to a square of the hydraulic diameter of the cell. Therefore, when the cell density is increased, the conveyance ratio of the components to be purified improves. However, pressure loss tends to increase in inverse proportion to the square of the hydraulic diameter. Therefore, there is a problem that with the increase of the conveyance ratio of the components to be purified, the pressure loss increases.
It is to be noted that the catalyst layer on the surfaces of the partition walls usually has a thickness of about several tens μm. Here, in a case where the components to be purified are diffused in the catalyst layer at an insufficient rate, the purification efficiency of the honeycomb catalyst tends to lower. This tendency is remarkable especially on low temperature conditions. Therefore, to increase the purification efficiency of the exhaust gas, in addition to the enlargement of the surface area of the catalyst layer, the thickness of the catalyst layer needs to be reduced to increase the diffusion rate of the components to be purified in the catalyst layer. Therefore, though there is an advantage that the increase of the cell density results in the enlargement of the surface area of the catalyst layer, there is still a problem that the pressure loss increases.
To reduce the pressure loss while increasing the purification efficiency of the exhaust gas, an inflow diameter of the honeycomb catalyst needs to increased, and a flow rate of the exhaust gas to be circulated needs to be lowered. However, when the honeycomb catalyst is enlarged, a mounting space for the honeycomb catalyst to be mounted on a vehicle or the like is limited, and it is sometimes difficult to mount the catalyst.
Patent Document 1: JP-A-2003-33664